1. Field of the Invention
The present invention relates to a reforming process of a quartz glass crucible for pulling up single crystal silicon by a ring-like arc discharge, which is especially suitable to reform the crucible having a large open diameter.
2. Discussion of the Background
When single crystal silicon is pulled up from molten silicon, a quartz glass crucible has been used as the vessel to take in the molten silicon. Conventionally, the crucible is produced by fusing a quartz powder using an arc flame. However, there are problems in that silicon oxide (SiO2, SiO, etc.) evaporated during the fusing is condensed again to fall down on the inside surface of the crucible and to form a foreign substance, or a metal powder (impurity), or a substance other than silica is adhered and included on the inside surface of the crucible to form a foreign substance. Moreover, a bubble included near the inside surface of the crucible is often expanded to burst to form an irregularity on the inside surface. When the foreign substance or irregularity exists on the inside surface of the quartz glass crucible, or the bubble exists under the inside surface, a reforming treatment, in which the inside surface is re-fused to remove the foreign substance or the bubble, has been carried out. Conventionally, in the reforming treatment, the arc discharge used for the production of the crucible is used as the means for heating the inside surface of the crucible.
The conventional electrode structure forming the arc discharge is a structure having 3 electrodes with 3-phase alternating current, in general, in which 3 electrodes are used, and the 3-phase alternating electric current flows to the 3 electrodes to form the arc (discharge) plasma between each of the electrodes. However, the structure has a fault in that the arc becomes unstable to be cut off when a distance between the electrodes is increased in order to expand the heating range. Especially, for a large crucible, when the influence of an air stream on the inside of the crucible created by the rotation of the mould becomes large, the arc is cut off easily in the conventional electrode structure.
Therefore, trials increasing the number of electrodes to expand the heating range have been carried out, and a structure having 6 electrodes with 6-phase alternating current has been proposed. However, for the structure having 6 electrodes with 6-phase current, as shown in FIG. 6, it is easy to generate an arc discharge between the electrodes facing each other, rather than between the electrodes neighboring each other, so that there is a problem that the heat of discharge at the central portion encircled by the electrodes becomes excessively large rather than the heat around the outside portion. Accordingly, it is difficult to heat uniformly the inside of the crucible.
Moreover, although the arc can be inclined in order to heat the wall portion, the arc becomes unstable easily and the device becomes complicated using the conventional electrode structure. In addition, although an oxyhydrogen flame or a plasma flame is suitable for a partial heating, the reformed face becomes non-uniform easily because of the partial heating, and the required treatment time is also long. Moreover, when the partial heating is carried out, it is necessary to also carry out annealing, and if the annealing is not carried out, there is a problem that the crucible is broken when cooled. Furthermore, as for the heating by an oxyhydrogen flame, a concentration of an OH group at the heating portion becomes high, so it is not preferable.